1. Field of the Invention
The invention is directed to a method for calibrating a SQUID gradiometer of an arbitrary order. More specifically, the invention is directed to calibrating SQUID gradiometers of an arbitrary order in a multi-channel SQUID system by inducing a magnetic field in such a manner so that the uniform field components at the pick-up and compensation coils of the gradiometer are equal to zero while the gradient field components are specifically known values.
2. Description of the Related Art
Superconducting quantum interference device (SQUID) gradiometers are used to measure weak magnetic fields and isolate the location of the field source. In particular, multi-channel systems of this type are utilized extensively in medical technology for measuring the magnetic fields emanating from the brain and heart. These magnetic signals are required for preparing magnetoencephalograms (MEG) and magnetocardiograns (MKG) in order to acquire the chronological curve of action streams of these organs and their spatial allocation.
Before the location of a field source can be isolated with the assistance of multi-channel SQUID gradiometer system, an exact calibration of each of the gradiometers must be obtained. An exact calibration entails a determination of the ratio between the magnetic flux density at the sensor coil (pickup coil) and the electrical voltage at the output of the drive electronics of the SQUID system. This ratio is referred to as the calibration factor of the gradiometer.
A known method for calibrating a multi-channel system of this species is described in IEE Transactions on Biomedical Engineering (in Press 1988) under the title "SQUID ARRAYS FOR SIMULTANEOUS MAGNETIC MEASUREMENTS; CALIBRATION AND SOURCE LOCALIZATION PERFORMANCE" by P. Costa Ribeiro, S. J. Williamson and L. Kaufman. This method is capable of obtaining a precision of approximately 2%, this precision being significantly greater than the 10% precision associated with the prior methods.
The method describes its use as applied to an ideal gradiometer that does not have any mis-match [or mis-balance] resulting from the uniform part of the applied field. A real gradiometer, however, has such a mis-match [or mis-balance]. This mis-match [or mis-balance] causes a voltage Vf at the output of the gradiometer resulting from the uniform part of the applied calibration field. The voltage Vf falsifies the measuring voltage V, the voltage Vf increasing in the case of second and higher order gradiometers.
Although it is in fact possible to calculate the voltage Vf if the mis-match factor f is known (it is possible to subtract it from the measuring voltage V), an exact measurement of the mis-match factor f is extremely complicated. Namely, a very uniform magnetic field must be generated and exact field o gradients up to the (n-1)th order must also be generated for gradiometers of the nth order.